3/62.4.2.2 Synchronous
Our idea of real-time computer conferencing differs from ordinary audio or video conferencing. Shared spaces, like electronic whiteboards, where participants of a computer conference can work together, can be essential. The idea goes back to the visionaries Engelbert and English and their NLS system [EE68] which provided a shared-screen mode for simultaneous collaborative authoring of structured documents [SG88]. The term WYSIWIS (What you see is what I see) is used for shared applications where all participants can see exactly the same thing.Pointing devices are also an important feature of shared applications. Viewing the pointers of all participants can be quite distracting. To minimise the confusion one could make pointers visible only on request [S+88]. Semantic telepointers [GGR96] change their image depending on the user and the task. They are supposed to give additional information.
Concurrency control is very important when objects can be modified by a number of people. In the Colab tools [S+88] a user gets a ``busy signal'' if another user is currently modifying an object. In the shared calendar RTCAL [SG88] users have to make reservations on an object before they are allowed to modify it.
The kind of media used in computer conferencing is highly dependent on the circumstances. If bandwidth allows, video will be very useful in meetings where feelings are often expressed by gestures. In fact, audio is more important than video. A slightly blurred video may be acceptable, whereas poor audio quality is often unacceptable [JB88]. When bandwidth is not wide enough, a small picture of a face in a corner of the screen may be enough to give the feeling of ``presence'' of the speaking person [Len97].